[SPARK-4348] [PySpark] [MLlib] rename random.py to rand.py

This PR rename random.py to rand.py to avoid the side affects of conflict with random module, but still keep the same interface as before.

```
>>> from pyspark.mllib.random import RandomRDDs
```

```
$ pydoc pyspark.mllib.random
Help on module random in pyspark.mllib:
NAME
    random - Python package for random data generation.

FILE
    /Users/davies/work/spark/python/pyspark/mllib/rand.py

CLASSES
    __builtin__.object
        pyspark.mllib.random.RandomRDDs

    class RandomRDDs(__builtin__.object)
     |  Generator methods for creating RDDs comprised of i.i.d samples from
     |  some distribution.
     |
     |  Static methods defined here:
     |
     |  normalRDD(sc, size, numPartitions=None, seed=None)
```

cc mengxr

reference link: http://xion.org.pl/2012/05/06/hacking-python-imports/

Author: Davies Liu <davies@databricks.com>

Closes #3216 from davies/random and squashes the following commits:

7ac4e8b [Davies Liu] rename random.py to rand.py

1 files changed, 1 insertions, 1 deletions

diff --git a/python/run-tests b/python/run-tests
index a4f0cac059..e66854b44d 100755
--- a/python/run-tests
+++ b/python/run-tests
@@ -72,7 +72,7 @@ function run_mllib_tests() {
     run_test "pyspark/mllib/clustering.py"
     run_test "pyspark/mllib/feature.py"
     run_test "pyspark/mllib/linalg.py"
-    run_test "pyspark/mllib/random.py"
+    run_test "pyspark/mllib/rand.py"
     run_test "pyspark/mllib/recommendation.py"
     run_test "pyspark/mllib/regression.py"
     run_test "pyspark/mllib/stat.py"